1. Field of the Invention
The present invention relates to a method of forming a hard carbon film over the inner surface of a guide bush to be in sliding contact with a rodlike workpiece, wherein the guide bush is mounted on an automatic lathe to support the workpiece for rotation and axial sliding at a position near a cutting tool (cutter).
2. Description of the Related Art
Guide bushes mounted on the column of an automatic lathe to hold a rodlike workpiece for rotation at a position near a cutting tool are classified into rotary guide bushes and stationary guide bushes. A rotary guide bush rotates together with a workpiece and holds the workpiece for axial sliding. A stationary guide bush remains stationary and holds a workpiece for rotation and axial sliding.
A guide bush of either type has a portion having a taper outer surface provided with slits to make the same portion elastic, a threaded portion to hold the guide bush on the column, and an inner surface for holding a workpiece. The inner surface always in sliding contact with a workpiece is liable to be worn and, particularly, the inner surface of a stationary guide bush is worn rapidly.
A guide bush proposed in JP-A No. 4-141303 has an inner surface to be in sliding contact with a workpiece, which slides and rotates on the inner surface, attached with a cemented carbide (super hard alloy sleeve) or a ceramic material attached to the inner surface by brazing or the like.
When the inner surface of a guide bush is attached with a cemented carbide or a ceramic material excellent in wear resistance and heat resistance, the wear of the inner surface of the guide bush can be reduced to some extent.
However, when the workpiece is subjected to heavy machining on an automatic lathe, in which the depth of cut is large and the cutting speed is high, the workpiece is damaged or seizing occurs due to decrease in the diametral clearance between the guide bush and the workpiece even if the inner surface of the guide bush is attached with a cemented carbide or a ceramic material, because the cemented carbide and the ceramic material have comparatively a large coefficient of friction and a low thermal conductivity. Therefore, it has been difficult to increase the depth of cut and cutting speed.
The stationary guide bush has advantages in that a workpiece can be accurately machined in a high roundness because the workpiece can be held so that its axis may not run out, less noise is generated, and the automatic lathe may be of a simple, compact construction.
However, the inner surface of the stationary guide bush is worn far more rapidly than that of the rotary guide bush and hence it is more difficult to increase depth of cut and cutting speed when the stationary guide bush is employed.
To solve this problem, we propose a guide bush provided with a hard carbon film over an inner surface thereof which is in sliding contact with a work piece, thereby drastically enhancing wear resistance on the inner surface and capable of increasing depth of cut and cutting speed without damaging the workpiece and generating seizing.
The hard carbon film is formed of a hydrogenated amorphous carbon closely resembling diamond in properties. Therefore, hydrogenated amorphous carbon is also called diamondlike carbon (DLC).
The hard carbon film (DLC film) has a high hardness (not lower than Vickers 3000 Hv), is excellent in wear resistance and corrosion resistance, and has a small coefficient of friction (about 1/8 that of a cemented carbide).
As a method of forming a hard carbon film over the inner surface of the guide bush, there is a plasma CVD method (Plasma Chemical Vapor Deposition Process) consisting of forming the hard carbon film by reducing the pressure to 5.times.10.sup.-3 torr adapted for forming the film in an atmosphere of a gas containing carbon, and applying a DC voltage of -3 kV from a DC power source to the guide bush to produce a plasma.
However in this plasma CVD method, since the gas containing carbon is decomposed to form the hard carbon film mainly using the plasma produced at the peripheral region of the guide bush, the hard carbon film can be uniformly formed over the outer surface of the guide bush but there is a problem in that the hard carbon film formed over the inner surface of the guide bush is inferior in adhesion, and the quality of the film with respect to, e.g., hardness is deteriorated.
This is caused by the fact that since there is a space in the center bore of the guide bush where electrodes of the same potential oppose each other, the plasma in the center bore generates an abnormal discharge called a hollow cathode discharge. The hard carbon film formed by the hollow cathode discharge is a polymeric film which is inferior in adhesion and is liable to be peeled off from the inner surface of the guide bush, and is low in hardness.
Further, in the method of forming the hard carbon film, a DC voltage of -3 kV is applied from a DC power source 73 to the guide bush 11 at 5.times.10.sup.-3 torr which is the pressure for forming the hard carbon film.
In a state where pressure in the vacuum vessel is about 5.times.10.sup.-3 torr, there are many electric charges such as electrons in the vacuum vessel interior, which makes the space impedance low. Accordingly, at the instant when the plasma discharge starts, an arc discharge which is an abnormal discharge is liable to occur in the guide bush.
Further, the time when the plasma discharge starts is the time when the hard carbon film is first formed, and hence the adhesion to the guide bush is influenced by the quality of the film formed at the first stage.
Accordingly, when arc discharge as abnormal discharge occurs at the first stage of the plasma discharge, the quality and adhesion of the hard carbon film are respectively deteriorated, which causes a problem that the hard carbon film peels off from the inner surface of the guide bush.
Accordingly, the present invention solves these problems, and it is an object of the invention to provide a guide bush provided with a hard carbon film, which is excellent in quality and adhesion, formed over the inner surface thereof which is in sliding contact with a workpiece.